pH — Acidity & Acid-Base Chemistry
Calculate pH, pOH, [H⁺], and [OH⁻]. Complete acid-base analysis with pH scale visualization.
Why This Scientific Calculation Matters
Why: pH measures acidity. It is logarithmic: each unit = 10× change in [H⁺]. Blood, oceans, and soils depend on pH.
How: Select calculation type (pH, pOH, [H⁺], [OH⁻]) and enter one value. The calculator derives the others.
- ●pH 7 = neutral
- ●pH + pOH = 14 at 25°C
- ●[H⁺][OH⁻] = 10⁻¹⁴
⚡ Quick Examples — Click to Load
Calculation Type
Input Value
🧪 Scientific Discoveries
pH scale invented 1909 by Sørensen (beer brewing)
— IUPAC
Blood pH 7.35–7.45; outside range can be fatal
— ACS
Lemon pH ~2 = 100,000× more H⁺ than water
— NIST
📋 Key Takeaways
- • pH = -log₁₀[H⁺] — pH measures acidity on a logarithmic scale from 0 (strongly acidic) to 14 (strongly basic)
- • pH + pOH = 14 at 25°C — knowing one automatically gives you the other
- • [H⁺][OH⁻] = 1 × 10⁻¹⁴ — the ion product of water (Kw) is constant at 25°C
- • pH 7 is neutral (pure water), pH < 7 is acidic, pH > 7 is basic
- • Each pH unit represents a 10× change in [H⁺] concentration — pH 3 has 10× more H⁺ than pH 4
💡 Did You Know?
📖 How pH Works
pH measures the concentration of hydrogen ions [H⁺] in a solution. The scale is logarithmic, meaning each unit represents a 10× change in concentration.
The Logarithmic Nature
A solution with pH 3 has [H⁺] = 10⁻³ = 0.001 mol/L. A solution with pH 4 has [H⁺] = 10⁻⁴ = 0.0001 mol/L — exactly 10× less acidic. This is why pH is so useful: it compresses a huge range (0.00000000000001 to 1 mol/L) into a simple 0-14 scale.
Water Autoionization
Even pure water contains some H⁺ and OH⁻ ions from autoionization: H₂O ⇌ H⁺ + OH⁻. At 25°C, [H⁺] = [OH⁻] = 10⁻⁷ mol/L, so pH = 7 (neutral). The product [H⁺][OH⁻] = 10⁻¹⁴ = Kw is constant.
Buffers & pH Stability
Buffers resist pH changes by absorbing excess H⁺ or OH⁻. Blood uses carbonic acid/bicarbonate (H₂CO₃/HCO₃⁻) to maintain pH ~7.4. Without buffers, adding a drop of acid would drastically change pH.
📐 Key Formulas
🎯 Expert Tips
💡 Quick pH Estimation
For [H⁺] = 0.001 M, count zeros after decimal: 0.001 has 3 zeros, so pH ≈ 3. For 0.00001 M, pH ≈ 5. Works for simple powers of 10!
💡 Temperature Matters
Kw changes with temperature. At 100°C, Kw = 10⁻¹², so pH + pOH = 12, not 14. Pure water at 100°C has pH = 6, but it's still neutral!
💡 Strong vs Weak Acids
Strong acids (HCl, H₂SO₄) fully dissociate: [H⁺] = concentration. Weak acids (CH₃COOH) partially dissociate: [H⁺] < concentration. Use Ka for weak acids.
💡 pH Indicators
Litmus turns red below pH 5, blue above pH 8. Universal indicator shows full rainbow: red (pH 1) → orange → yellow → green (pH 7) → blue → purple (pH 14).
⚖️ Common Substances pH Reference
| Substance | pH | [H⁺] (mol/L) | Classification |
|---|---|---|---|
| Battery Acid | 0.0 | 1.0 | Strongly Acidic |
| Gastric Acid | 1.5 | 3.2×10⁻² | Strongly Acidic |
| Lemon Juice | 2.0 | 1.0×10⁻² | Acidic |
| Vinegar | 2.5 | 3.2×10⁻³ | Acidic |
| Coffee | 5.0 | 1.0×10⁻⁵ | Acidic |
| Milk | 6.5 | 3.2×10⁻⁷ | Slightly Acidic |
| Pure Water | 7.0 | 1.0×10⁻⁷ | Neutral |
| Blood | 7.4 | 4.0×10⁻⁸ | Slightly Basic |
| Baking Soda | 9.0 | 1.0×10⁻⁹ | Basic |
| Soap | 10.0 | 1.0×10⁻¹⁰ | Basic |
| Ammonia | 11.6 | 2.5×10⁻¹² | Strongly Basic |
| Bleach | 13.0 | 1.0×10⁻¹³ | Strongly Basic |
❓ Frequently Asked Questions
What is pH?
pH stands for "potential of Hydrogen" and measures the acidity or basicity of a solution. It's defined as pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. The scale ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral.
Why does pH + pOH = 14?
This relationship holds at 25°C because [H⁺][OH⁻] = Kw = 1 × 10⁻¹⁴. Taking negative logarithms: -log([H⁺]) - log([OH⁻]) = -log(10⁻¹⁴), which simplifies to pH + pOH = 14. At other temperatures, Kw changes, so the sum changes too.
Can pH be negative or greater than 14?
Yes! For very concentrated acids, pH can be negative (e.g., 10 M HCl has pH ≈ -1). For very concentrated bases, pH can exceed 14 (e.g., 10 M NaOH has pH ≈ 15). However, the 0-14 range covers most practical applications.
What's the difference between strong and weak acids?
Strong acids (HCl, H₂SO₄, HNO₃) completely dissociate in water: HA → H⁺ + A⁻. Weak acids (CH₃COOH, H₂CO₃) partially dissociate, with an equilibrium constant Ka. For strong acids, [H⁺] = concentration. For weak acids, you need Ka to calculate [H⁺].
How do buffers work?
Buffers contain a weak acid and its conjugate base (e.g., CH₃COOH/CH₃COO⁻). When H⁺ is added, the base absorbs it. When OH⁻ is added, the acid neutralizes it. This keeps pH relatively stable, which is critical for biological systems like blood.
Why is logarithmic scale used for pH?
Hydrogen ion concentrations span an enormous range (from 1 M to 10⁻¹⁴ M). The logarithmic scale compresses this into manageable numbers (0-14). It also makes sense chemically: pH 3 feels "one unit more acidic" than pH 4, even though [H⁺] differs by 10×.
What happens to pH when temperature changes?
The ion product of water Kw increases with temperature. At 100°C, Kw = 10⁻¹², so pH + pOH = 12. Pure water at 100°C has pH = 6, but it's still neutral because [H⁺] = [OH⁻]. Neutral pH is temperature-dependent!
How is pH measured experimentally?
pH meters use a glass electrode that responds to [H⁺]. The voltage difference between the electrode and a reference electrode is proportional to pH. pH paper uses indicators that change color at specific pH ranges. Universal indicator shows a full spectrum of colors.
📊 pH by the Numbers
📚 Official Data Sources
⚠️ Disclaimer: This calculator provides estimates for educational purposes. Real solutions involve activity coefficients, temperature effects, ionic strength, and buffer capacity. For precise measurements, use calibrated pH meters. Always follow safety protocols when handling acids and bases — wear protective equipment and work in well-ventilated areas.
⚠️For educational and informational purposes only. Verify with a qualified professional.